The melt bath in open metallurgical vessels used in the steel industry, for example in continuous-casting tundishes, is usually covered with a covering agent or covering material which melts and forms what is known as the top slag. The top slag ensures a liquid protective layer on the metal bath surface, which is intended to maintain the oxidic purity level and prevents gases from being taken up out of the atmosphere and absorbing non-metallic inclusions from the melt.
Top slag agents for covering a melt bath generally have a melting point that is typically 150° C. below the liquidus temperature of the melt, so that they melt a short time after being applied.
The composition of the top slag depends on the requirements of the metallic melt bath. For steel making, it is mostly basic covering agents based on calcium aluminate, e.g. C12A7 (12 CaO.7 Al2O3) that are used. These may, for example, be mixtures or melt products from an Al2O3 carrier, such as bauxite or alumina, and a CaO carrier, such as limestone, calcined lime or dolomite. However, it is also possible, for example, to use continuous-casting powders as a mixture of SiO2, CaO, Al2O3, fluorine or soda components or vanadium slag-forming agents.
The liquid slag layer dissipates large quantities of heat outward from the melt bath and therefore causes high heat losses. To prevent this, a thermal barrier agent is applied as a coating to the slag. The thermal barrier agents should not melt at the melt bath and slag melt temperatures and should be sufficiently inert or nonreactive for them not to participate in the metallurgical process. By way of example, one thermal barrier agent used is biogenic silica in the form of rice husk ash. Furthermore, granulated spray-dried granules which are in the form of hollow minispheres are used.
In practice, the metal melt is generally first of all covered with the top slag agent; then, the thermal barrier agent is added to the molten top slag. The combination of top slag plus thermal barrier agent is also known as a “sandwich covering”.
One drawback of this known sandwich covering method with thermal barrier is that two different agents have to be used. Two products have to be kept in stock, and it must be ensured that they are not used in the wrong order on site. Another drawback is that reactions still occur between the dry, more solid thermal barrier agent and the liquid top slag, and these reactions have an adverse effect on the metallurgical effect of the top slag. By way of example, SiO2 from the rice husk ash can be taken up by the top slag until the saturation limit is reached, with the result that the top slag releases oxygen to the metal melt, something which the top slag is actually supposed to prevent.
Melts in metallurgical casting ladles are often also covered with thermal barrier agents. The melt bath surface of a casting ladle is covered with the thermal barrier material, for example at the end of the metallurgical work, thereby reducing the thermal losses.
In some cases, a thermal insulation is even applied before the metallurgical work has ended, for example if relatively long transfer or standing times are intended from tapping to the next treatment stage. In this case, the thermally insulating covering may have to be removed again by slag removal prior to the next treatment stage, since it would impede the subsequent metallurgical work of a top slag applied subsequently. This measure requires additional outlay, and considerable delays the metallurgical work and leads to not inconsiderable losses of material in terms of the thermal barrier agent.
It is an object of the invention to ensure good metallurgical work by a covering agent for a metallurgical melt bath and simpler thermal insulation.